Continuous casting mold level control

ABSTRACT

A molten metal continuous casting mold level control system is disclosed employing an inductance detector positioned within a water cooled continuous casting mold. The detector is mounted in the jacket of the mold about the generally hollow cylindrical mold core which core includes an electrically conductive form having a billet shaping surface. The output voltage from the detector is bucked against a signal that is related to an input signal, filtered and amplified to derive a signal whose polarity from a reference level and magnitude are proportionate to the direction and magnitude of change in the steel level. This signal is used to control the billet drive to correct changes from the desired level.

United States Patent Crowell et al.

[54] CONTINUOUS CASTING MOLD LEVEL CONTROL 834,783 5/1960 Great Britain ..l64/282 Primary Examiner-R. Spencer Anneal- [72] Inventors: Cmweu Cedar Lani Ind; Jam's Auom -Donald W. Banner William S. McCurr and John I R. Tomashek, Wood Dale; Donald H. f y

Ward, Glen Ellyn, both of ill. [73] Assignee: Borg-Warner Corporation, Chicago, Ill. [57] ABSTRACT [22] Filed: Jan. 16, 1970 molten metal continuous casting mold level control system IS disclosed employing an Inductance detector positioned PP 3,421 within a water cooled continuous casting mold. The detector is mounted in the jacket of the mold about the generally hollow cylindrical mold core which core includes an electrically i "164/l54 5 5? conductive form having a billet shaping surface. The output 58 H L 4 273 282 voltage from the detector is bucked against a signal that is re- I 1 e o are lated to an input signal, filtered and amplified to derive a signal whose polarity from a reference level and magnitude [56] References Cited are proportionate to the direction and magnitude of change in UNITED STATES PATENTS the steel level. This signal is used to control the billet drive to correct changes from the desired level. 3,519,060 7/ 1970 Vischulis ..l64/155 4 Cla lms, 3 Drawing Figures FOREIGN PATENTS OR APPLICATIONS 693,267 6/1953 Great Britain.. 164/82 [5, Q Q Molten o l Signal Source Steel [32 1 al I Input Signal mducflve agfT 'f e I I I Level 0 t t SI nol Phase l u g 1 Detector x30 Shifter I 59 0 f I .4 r p Continuous I 3?? I I M Id I Ch Mold 0 eve on e Bucker Amplitude sign l (Wmer mks) \Ter 38 22 o...

l e 143, I Billet 42 l-40 Drive b le Motors I I45 Phase I 24 I I DIII/e I Detector 8| L Drive Cooling Si L Demodulotor J Control Shearing "f vMc]d Level Change I ll Amplitude 8r Directlon Finished Steel Billers 1 CONTINUOUS CASTING MOLD LEVEL CONTROL BACKGROUND OF THE INVENTION In one commercial important process of continuously casting steel, a stream of molten metal is poured from a tundish box into a water-cooled mold. The steel is colled sufficiently within the mold that it may be withdrawn as continuous billet. The level of molten steel in the mold is determined by the rate of flow of the molten steel from the tundish and the rate of withdrawal of cast billet by a drive system. The present invention concerns improved level detection and control in this environment. The position that a continuous casting machine may occupy in one particular steel mill is noted in the article entitled Steel by D. R. G. Davies in the McGraw-I-Iill Yearbook of Science and Technology (1969) at page 325.

Basically, a continuous casting mold functions to take liquid molten steel and to transform it into a continuous billet of steel by passing it through a water-cooled mold that forms it and cools its outer surface to a solid state. Thereafter, the continuous billet is further cooled and cut or sheared into individual steel billets. For a general source on continuous steel castings, reference could be had to the works: The Continuous Casting of Steel in Commercial Use by K. P. Korothov, H. P. Mayorov, A. A. Skvortsov and A. D. Akimenko translated by V. Alford; Continuous Casting of Steel by M. C. Boichenko 1957) translated by L. Herdan and R. Sewell; and Continuous Casting D. L. McBride in the Proceeding of Technical sections of the Iron and Steel Division of the Metallurgical Society of the American Institute of Mining, Metallurgy and Petroleum Engineers (Autumn, 1961).

It should be readily apparent that the working of such a product as molten steel is both extremely dangerous and difficult. In the case of forming it into continuous billet by the continuous casting method, it is desirable to maintain the level of the molten steel in the casting machine within fairly close tolerances. If the level is maintained too high as by pouring excessive amounts of steel in the machine, it may splash over causing loss of steel, perhaps damage to equipment, and possibly danger to the human operators. In addition, if the molten steel is too low in the mold it may have insufficient exposure to the cooling walls to solidify, again resulting in breakout. This can occur at a point somewhat below the end of the mold as the continuous billet normally is hardened or solidified only at its outer surface and contains a molten core. Also if the steel is extracted too slowly or remains in the mold too long, the steel may harden too much making it difficult to handle in later stages or, in an extreme case, solidifying within the mold itself. To overcome the tendency of the molten steel to adhere to the shaping surface of the mold or form, it is the normal practice to both add a lubricant, a special oil, at the top of the mold and to oscillate the mold fairly rapidly in the vertical direction. This rapid up-and-down movement, although improving the overall system, makes the problem of detecting and controlling mold level height more difficult.

The importance of maintaining a proper level in the mold has not been overlooked by those familiar with this industry. Complicated devices and complex and expensive machinery have been employed to detect and maintain the proper level. One presently used commercial mold level indicator and control uses a radioactive source such as cesium 137 sealed in a stainless steel capsule which is encased in Mallory I000 metal for radiation shielding and, which in turn, is housed in a steel cabinet to protect the Mallory metal from molten steel splashover damage. This source is positioned adjacent to but spaced from the mold, on one side thereof, to direct a beam of radioactive particles through the mold mechanism, its water jacket, the molding core itself, and any molten steel therein. A solenoid operated shutter mechanism is movably provided to interrupt the radiation beam, to provide safe access for the operating personnel around the radioactive source. The radiation is picked up by a detection unit comprising a solid state high-sensitivity radiation detector of the scintillation type enclosed in a separate water-cooled steel housing positioned outside of but adjacent to and spaced fromthe mold unit on the opposite side from the source. Normally, two detection units are employed for monitoring molten steel levels, one for normal operation and one for start-up. The signal from the detector is used to vary the rate of speed of the drive mechanism that removes the continuous billet from the steel casting mold.

Despite the shielding and precautions taken with this radioactive source, it has often proved to be unable to withstand the extreme environment in which it is used. It has been the experience of those practicing this steel making process that the occasional and unavoidable spills and splashes of molten steel have caused. the radioactive detecting units to fail or be taken out of operation for repairs for extended periods of time. It has been found to be both expensive and time consuming to attempt to repair and maintain in operable condition such units.

When the units are disabled, the most widely employed alternative is human supervision. That is, a worker or operator in protective clothing and shielding is detailed to physically inspect the continuously changing levels of molten steel in the continuous casting machines and to vary the flow of steel to the machine and from machine, based upon his human judgment and vision. Needless to say, this environment, which is extreme for a shielded radioactive detecting system, is far from ideal for the human worker detailed to such a task. Indeed, it is not surprising to find that these workers often make errors in judgment resulting in less than optimum performance and malfunctioning of the continuous casting machinery.

SUMMARY OF THE INVENTION A continuous casting and mold level control system, constructed in accordance with the present invention, includes a mold unit for receiving molten metal, which unit has a core including an integral electrically conductive form which defines a shaping surface into which the metal may pour and from which billet is produced. A jacket surrounds the form for containing a circulating electrically conducting coolant. An inductance level detector is positioned :in the jacket adjacent to but outside of said core and substantially surrounded by the coolant is provided coupled to a source of excitation signals and means for extracting a signal indicative of the level in the mold from the detector, and for determining a control signal from it. The billet drive is controlled from that signal such that a change in the molten level results in a change in the rate of billet extraction so as to restore the level to a predetermined level.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a block diagram illustrating a mold level detection and control system incorporating the present invention;

FIG. 2 is a perspective view partially cut away to show interior parts of a continuous steel casting mold, including inductive detector of the type that may be used in the system of FIG. 1;

FIG. 3 is a circuit diagram of the system of FIGS. 1 and 2.

DETAILED DESCRIPTION Referring to FIG. 1 there is depicted a steel billet continuous casting system generally designated 10 which is constructed in accordance with the principles of the present invention. The system 10 functions to transform molten steel into steel billet. A molten steel source 12 (which may be an electric-arc furnace feeding a tundish) feeds molten steel to a continuous casting mold, generally designated 20, from which it emerges as a continuous hot billet 14 which is advanced therefrom by a drive 16. The billet 14 is more fully cooled and sheared into billets at a later station 18. For cooling the molten steel, the mold 20 is provided with a water jacket into which relatively cold water is pumped and from which relatively hot water extracted during the casting process.

The billet drive 16 is driven by drive motors 22 which are controlled from a drive control unit 24 in a manner that allows the speed that the system 16 drives the billet 14 to be varied over a range of speeds. As the rate of molten steel from the source 12 is reasonably constant and not subject to fine control the speed of the drive 16 is the primary determiner of steel flow through the mold 20 during normal operation.

In accordance with the present invention an inductive leveldetector, generally designated 30, is provided as a part of the mold unit 20 and a drive control signal circuit, generally designated 40, is provided.

The circuit 40 comprises an ac signal source 41, which may be the conventional 60 cycle power lines or some other frequency signal source which is either higher or lower in frequency. This source 41 is coupled, as indicated by line 31, to the level-detector 30 and, as indicated by line 32, to a phase shifting circuit 35. The output of the phase shifting circuit 35 is fed to a bucking circuit 36 as indicated by line 37. The output of the level detector is fed to the bucking circuit as indicated by line 39. The combined output from the detector 30 and circuit 35 is preferably nulled or balanced by circuit 36 to be zero at desired height or level of the molten steel within the mold 20. Thus, whenever output is fed over the line 38, it is representative of the shift in the output signal resulting from a change of the level in the continuous mold 20. This output 38 is amplified by a conventional amplifier 41 and fed, as indicated by line 42, to a phase detector and demodulator 45. The output from the phase shifter 35 is also fed, as signified by line 43, to the phase detector and demodulator 45 wherein the relative phase between the outputs on line 43 and 42 are compared and detected and a demodulated dc signal is produced on the output line 47 and fed to the drive control 24.

The output on line 47 is preferably a dc voltage whose change in amplitude from a nominal voltage level represents the magnitude of change of height of the molten steel in the continuous mold 20 and also whose direction of change (plus or minus), from that nominal level, represents the direction of change in steel height (up or down). This signal is fed, as symbolized by the line 47, to the drive control unit 24 which in turn controls the drive motors 22 to alter the speed of withdrawal of the hot billet 14 from the mold so as to bring the level of molten steel therein back to the desired level.

Referring now to FIG. 2 there is depicted one type of mold unit 20 including a level detector 30 constructed in accordance with the present invention. The mold 20 is generally cylindrically shaped and is positioned above and below a mounting platform 20? of a generally square shape. In use the platform 20! lies in the horizontal plane and the generally cylindrical shaped mold extends vertically there through. The interior steel-molding surface is formed from a generally square-shaped, in crosssection, form 20A made of a unitary high thermal-conductive material, such as copper. Positioned about the copper form 20A is a water jacket of a generally hollow tubular shape designated 20]. This jacket 20J includes a top piece 20T and a bottom piece which together define an elongated annulus-shaped chamber 20C. To provide proper cooling for the fonn 20A, a generally square-shaped, in crosssection, sleeve 208 is provided within the chamber 20C about the member 20A. The sleeve 208 is so sized as to allow circulation of water between it and the copper form 20A and it is preferably made of stainless steel.

The inductance level detector 30 for detecting the level of molten steel 13 within the copper form 20A comprises a pair of coils wound in and insulated within an epoxy or other heatresistant electrically insulating material, and formed into a generally square-shaped, in cross-section, hollow cylindrical sleeve for fitting over the stainless steel sleeve 205, within the chamber 20C. The copper form 20A and its sleeve 208 form the core 20AS of the mold 20. The coils wound in the inductor detector 30 are wound with successive turns lying approximately horizontal to have the coils central axes vertical so as to coincide with the vertical central cylindrical axis of the mold 20 and the direction of change of molten steel therein. The inductance detector 30 is positioned at about the height at which it is desired to maintain the molten steel 13 and, thus, need not extend the full length of the mold 20. An output cable 33, also insulated, is provided between the unit 30 and a water proof exit port 2015.

As is conventional for such molds as the mold 20, guides 206 are provided passing through corresponding sleeves in the support platfonn 20F to allow for vertical motion of the mold 20 during use. Further provided water inlet pipes 20W and water exit pipes 20X to communicate water coolent to the chamber 20C. The chamber 20C is normally filled with water which functions to cool the copper form 20A as well as the steel sleeve 20S and here additionally functions to cool the windings of the inductance detector unit 30. The particular mold unit 20 depicted in FIG. 2 is a modification of a Koppers Company, Inc. of Pittsburgh, Pennsylvania, Continuous Casting Billet Machine. The only modification to this standard mold unit has been the inclusion of the detection unit 30 and its corresponding outlet 20E within the water jacket chamber 20C. It has been found in operation that the presence of the unit 30 does not affect the cooling rate or the operational efficiency of the mold 20 to any measurable degree.

Referring now to FIG. 3, the system 10 and especially the control circuit 40, will now be explained in more detail. From an ac line voltage power source 51, line 51A and 51C are connected to the ac power lines which may for example be 118 volts alternating current, such as commonly available in the United States, or may be some other alternating current source. The line 51A is coupled through a safety fuse 52 and an on-off switch 53 to one side of a primary coil of a stepdown transformer 54. The line 51C is coupled to the other side of the transformer 54.

One side of the secondary coil of the transformer 54 is connected to a conduction line 31A while the other side of the secondary coil of the transformer 54 is connected to a conduction line 318. Between the lines 31A and 31B is preferably connected an indicating lamp 55. The lamp 55 as well as the primary actuating on-ofi switch 53 is preferably placed upon a control panel located remotely from the continuous casting mold 20. The line 31B is also connected through a resistor 56 to the line 31C. The lines 31A and 31C form the signal input 31 to the primary coil of 30C of the induction level detector Connected to the junction of the resistor 56 and line 31C is a conductor 32C and also connected to the junction of the resistor 56 and the conductor 31B is a conductor 32A. Between the lines 32A and 32C is connected the primary coil of a transformer 58. The secondary coil of the transformer 58 is center tapped. This center tap is connected to one end of the primary of another transformer 59. The other end of the primary coil of the transformer 59 is connected, through a capacitor 61, to one end of the secondary coil of the transformer 58 and also through a resistance circuit 62 to the other end of the secondary of the transformer 58. This resistance circuit 62 comprises a fixed resistance 62F in parallel with a variable resistance 62V which parallel connection is in series with a third resistance resistor 628.

The secondary coil on the transformer 59 has two intermediate tap points 63 and 64. The tap 63 is a center tap. Between these two tap points 63 and 64 the bucking signal is derived.

This voltage is used as a bucking voltage for cancelling out or nulling the output from the secondary detector coil 305. To be able to adjust the amplitude of this bucking voltage, it is applied to a potentiometer 72 and a fixed resistance 71. The position of the tap on 72 determines the magnitude of bucking voltage introduced in series with the secondary detector coil 308.

The voltage taken from the tap of the potentiometer 72 is reduced in magnitude and shifted in phase from the primary signal impressed on the primary coil 30P of the detector 30. This reduced and phase shifted bucking voltage from the tap of the resistor 72 is connected to the line 39A which is connected to one end of the secondary coil 308 of the inductance detector 30. The other end of the coil 308 is connected to the line 39C and thereafter coupled to a band pass filter circuit 75 tuned to the signal source frequency. This filter 75, forms part of the amplifier 41.

The band pass filter 75 is connected to an adjustable voltage source power supply via line 76 and the filter output is connected to a line 77. The line 77 is connected to the positive primary input of an operational amplifier 80. The operational amplifier 80, which may be a motorola MC14336 type, is a well-known active circuit element which is here connected to serve as an ac low-frequency amplifier. The operational amplifier 80 has the line 77 from the band pass filter 75 connected to its primary positive input while its negative primary input is connected to a tapped resistor 78. One of the fixed terminals of the resistor 78 is connected to ground and the other is connected through a feedback resistor 79 to the output terminal of the operational amplifier 80.

In addition, the operational amplifier 80 has its conventionally numbered 6 and 8 terminals connected to source of positive voltage B+ from the power supply 70 over a line 81, has its No. 9 and No. terminals inter-coupled by a capacitor 82, has its output coupled through a capacitor 83 to its No. 3 terminal, and has its'No. 4 terminal connected to a source of negative dc potential from the power supply 70 via the line 85. The amplified ac signal from the operational amplifier 80 is capacitively coupled, via a capacitor circuit 87, to a line 42A which is connected to one side of the primary coil of a transformer 88. The other side of the primary of the transformer 88 is connected to ground.

The transformer 88, which forms part of the phase detector demodulator circuit 45, has a center tapped secondary coil, one end of which is connected through a single pole, double throw switch section 89, to a line 91. This line 91 is connected to a phase sensitive demodulator circuit generally designated 90. The opposite side of the circuit 90 is connected through a line 92 to another single pole, double throw switch section 93, to the other end of the secondary of the transformer 88. Across the other inputs of the bridge circuit 90 is connected the full secondary of the transformer 59 via the lines respec tively designated 43A and 43C. Over these lines 43A and 430 are coupled the ac signal, indicated in FIG. 1, by line 43.

The bridge 90 is, more particularly, comprised of four solid state diodes 90A, 90B, 90C, 90D and four resistors 90W, 90X, 90Y, and 90Z. With a diode and a resistor connected in series forming each of four branches of the circuit 90. That is, the anode of the diode 90A is connected to the line 43A, while its cathode is connected through the resistor 90W to the line 91. Also the line 91 is connected through the resistor 90X to the line 91. Also the line 91 is connected through the resistor 90X to the anode of the diode 908 whose cathode is respectively connected to the line 43C. The diode 90C has its anode connected to the line 43C and its cathode connected through the resistor 90! to the line 92. And the line 92 is connected through the resistor 90Z to the anode of the diode 90D whose cathode is in turn connected to the line 43A.

The variable level direct current output from the bridge circuit 90 is taken from the center tap of the secondary coil on the transformer 88 via line 94 which is connected to a filter or smoothing circuit 95. The circuit 95 includes a first resistor 96 connected to the line 94 and to one side of a capacitor 97 whose other side is grounded. The junction between the resistor 96 and capacitor 97 is, in turn, connected through resistor 98 to a second smoothing capacitor 99 whose other side is likewise grounded. The junction between the resistor 98 and capacitor 99 is connected through a resistor 101 to ground and also connected to the output line 47 between which line and ground the drive control signal is developed.

The other side of the direct current output from the bridge circuit 90 is taken from the center tap 63 of the secondary coil on transformer 59. It is taken over conductor 93 to the tap of potentiometer 67. A dc voltage developed across power supply diodes 129 is applied to the network made up of potentiometer 71', potentiometer 67, and resistor 68. The settings of 71 and 67 determine the magnitude of dc voltage added in series with the output of the bridge circuit between center-tap 63 and the ground reference plane.

The tenn ground plane is here used in the conventional electronic circuit sense and is not necessarily earth potential.

Also depicted in FIG. 3 is a read out and balancing meter which serves a dual function of indicating the output voltage during normal operation and also aiding to achieve a balance null during set-up of the system 10. The meter 100 is a microammeter. One of its input terminals is connected through a current limiting resistor 103 to the line 47 and its other terminal connected through a single pole, double throw switch section 105 to ground. As such the meter 100 gives a direct reading of voltage output of the system. The meter 100 is preferably mounted on the control console for view by an operator.

In its balancing function, the meter 100 is connected through the other switch position of the switchsection 105 to the center tap line 94 and is further connected through re sistance network including the resistor 109 to the cathodes of two diodes 111 and 112. The anodes of the diodes 111 and 112 are respectedly connected to the second terminals of the switch sections 89 and 93. The switches 89, 93 and 105 are preferably ganged together so as to be thrown at the same time, as indicated, in part, by the line 113. When the switch sections 89 and 93 are thrown they connect the ends of the secondary coil of the transformer 88, through their respective diodes 111 and 112 and the resistor 109 to the microammeter 100. The other side of the meter 100 is connected to the center tap of the secondary of the transformer 88 via the line 94.

This circuit forms a full wave rectifier of the output of any ac voltage induced from the amplifier circuit 41. As at balance, we have specified that the bucking voltage connected from the taps 63-64 of the transformer 59 should cancel out the induced voltage of the secondary coil 30S of the inductive level detector. Thus, in a balanced condition no current would be produced by the full wave rectifiers and thus reading zero deflection on the meter 100 would be expected. The meter 100 may then be used to set up or balance the system 10 by adjusting various variable circuit elements to zero the meter 100, for the desired steel height in the mold 20. When so used the meter 100 is effectively functioning as an ac volt meter. As balance is approached, the sensitivity of the meter 100 may be increased by decreasing the effective resistance of the circuit as by adding the additional resistor 116 in parallel with the resistor 109 by depressing the sensitivity push-button switch 117.

The power supply 70 will be briefly described at this point. While the power supply 70 is shown in detail, it should be understood that any equivalent power supply may be employed. It includes the transformer 120 whose primary coils are coupled across the primary of the transformer 54 and the ac power lines 51. The output or secondary of the transformer 120 is connected across a full wave rectifying bridge 121 consisting of four diodes, whose output is connected to lines 122 and 123. A smoothing capacitor 124 is connected between the lines 122 and 123. Also connected to the line 123 is a resistor whose other end is connected through a capacitor 126 to line 122. The capacitor 124 and 126 with resistor 125 function as a smoothing or filtering circuit. The line 122 is also connected through a voltage regulating zener diode 127 to ground, while the junction of the resistor 125 and the capacitor 126 is connected to a voltage regulating zener diode 128. This junction point forms the primary B+ output line. The line 122 forms a primary B- output line. The junction of the resistor 125 and 126 is connected to the cathode of a zener diode 128, whose anode forms the low level B'+ source and is further connected to the anode of the first of five voltage regulating diodes 129 that are series connected, anode to cathode, between the anode of the zener diode 128 and ground. An adjustable bias voltage input for the amplifier 80 through the filter 75 is formed by the voltage dividing resistor network 130 which includes a first resistor 131 connected to the B- line and one end of potentiometer 132 whose other end is connected through a resistor 133 to the B+ line. The arm on the potentiometer 132 is connected to the line 76.

OPERATION OF FIGURE 3 As mentioned above, in overall operation the circuit of FIG. 3 produces a direct current voltage on line 47 which do voltage varies from a nominal level in magnitude and direction corresponding to the magnitude and direction of change in the height of molten steel within the mold 20. The input signal source 41 couples an ac signal of line frequency to the lines 31A and 31C and thus impresses that signal upon the primary coil 30?. A signal proportionate to this primary coil 30P signal is developed across resistor 56 and thus coupled to the transformer 58 and from this to the transformer 59. Adjustment of the variable resistor 62V in the resistance circuit 62 allows for shifting the phase of the output of the transformer 59. Thus, the transformer 59 provides, between its tap points 63 and 64, an alternating current signal of reduced magnitude and shifted-phase from that of the input signal developed in the primary 30P.

Whenever a signal is developed in the secondary 308 it has to buck the signal picked off at point 72. The harmonics of the fundamental frequency injected into the primary, such as may be generated in the secondary, are eliminated by the band pass filter 75 so that only fundamental frequency signals are presented to the operational amplifier 80. With a deviation from the desired molten steel level, a small ac signal of the fundamental frequency will be present on line 77. This ac signal is amplified by the operational amplifier 80 coupled through the dc blocking capacitance 87 and coupled through the transformer 88.

The center tap 94 of the transformer 88, in relationship to ground, developes a nominal signal in the absence of any ac signals by the setting of the potentiometer 67 which governs the dc potential at the center tap 63 of transformer 59. This may, for example, be set for a nominal value of two volts.

The demodulator 90 functions to develop a net negative or positive voltage at the line 94, relative to center tap 63, from the ac signal amplified by the amplifier 80 and coupled thereto by the transformer 88 and lines 91 and 92 whenever the coupling between the coils 30P and 308 is more or less than that existing at the desired molten steel height.

The output voltage at line 94 with respect to ground is thus the dc value at center tap 63 established by the setting of potentiometer 67, plus the positive or negative output from the demodulator 90. The dc level established by the setting of 67 determines the set point or operating level, while the output from the demodulator provides a corrective signal to adjust the drive control unit 24 and maintain a constant steel level in the mold.

It should be pointed out that the system is relatively insensitive to changes in input signal level from the power source 51 or to changes in primary coil or circuit resistance 30, 31. This is because the bucking voltage from transformer 59 is derived from transfonner 58 which receives its excitation from the voltage drop across resistor 56. Thus, if the line voltage 51 should drop or the resistance of coil 30? should increase, the current in 30? decreases and the induced secondary voltage in 308 also decreases. However, a reduction in current in 30! also reduces the voltage across resistor 56 and thereby reduces the bucking voltage developed from transformer 59. Thus, there is no net change in the signal to filter 75 and operational amplifier 80, and the level of steel in the mold is unaffected by change in line voltage 51 or changes of coil resistance 30F.

It should now be apparent that a new and improved mold level control system of an advantageous type has been described. Although the mold level control system of the present invention have been described in the process of continuously casting steel, which is the best mode presently known for practicing the invention, it will be obvious to those skilled in the art that it may be employed in continuous casting of other metals and materials. It may also find other uses in other fields.

Various of the features of the invention have been particularly shown and described, however, it should be obvious to one skilled in the art that various modifications may be made therein without departing from the scope of the invention.

What is claimed is:

1. In a mold level control system for controlling the level of molten metal in a continuous casting mold of the type that has a core defining a shaping surface, which core includes a layer of highly conductive material and which mold receives molten metal from a source and from which a continuous billet of at least partly hardened metal is extracted by a variable speed drive system, the improvement of having:

an inductance mold level detector, including at least one inductance coil, which detector is positioned adjacent to but without the mold core, for detecting variations in the level of molten metal therein;

means coupled to said mold level detector and to the billet drive system for controlling the speed of the billet drive system in response to said detector so as to vary the speed thereof to maintain, during normal operation, the level of the molten metal within a desired predetermined range;

said inductance mold level detector comprising two coils,

said one coil and a second coil; and

said controlling means including:

a signal source coupled to said one coil for exciting it with an ac input signal,

means coupled to said second coil for deriving an output signal therefrom which is related to the input signal and to the level of molten metal in the mold;

a source of a second signal related to the input signal but phase shifted therefrom, and

means coupled to said output signal deriving means and said second signal source for deriving a dc mold level control signal whose amplitude varies in proportion with changes in the level of molten metal in the mold core from a predetermined level therein;

whereby the mold level control signal may be employed to govern the operation of motors in the billet drive system to counter changes in the molten metal from the predetermined level.

2. A continuous steel casting system comprising:

a source 12) of molten steel;

a mold unit (20) having a generally vertical core including an integral electrically conductive form defining a shaping surface (20A) into which form said source pours molten steel at an approximately constant and continuous rate, and a jacket about said form in which an electrically conducting coolant is circulated and from which mold unit a continuous steel billet is produced;

an inductance level detector (30) positioned in said jacket adjacent to but outside of said core substantially surrounded by said circulating electrically conducting coolant to detect the level of molten steel in the form;

a billet drive system (16) that is responsive to a drive control signal to vary the rate of removal of the billet from the mold unit;

a source (41) of excitation signals coupled to said inductance detector; and

means (40, 47, 24, 22) coupled to said inductance level detector for extracting therefrom a signal indicative of the level of molten steel and for determining therefrom a variable drive control signal therefrom, and for coupling that control signal to said billet drive system;

whereby a change in the level of molten steel results in a change in the rate of billet extraction so as to restore the level to a predetermined level.

3. The invention of claim 2, wherein:

said inductance level detector within said mold unit includes two coils, a primary coil and a secondary coil;

said source of excitation signals are coupled to said primary coil;

said means for extracting a signal from said inductance level detector and for determining a control signal therefrom is coupled to said secondary coil and includes means for producing a second signal related to but altered from said excitation signal and means for combining the extracted signal from said secondary coil and said second signal to produce the control signal.

4. A continuous metal casting system comprising:

a source (12) of molten metal;

a mold unit (20) having a generally vertical, integral and electrically conductive form, which form defines a shaping surface (20A) and into which form said source pours molten metal and from which form a continuous steel billet is produced, and a jacket about said form in which an electrically conducting coolant :is circulated;

an inductance level detector (30), positioned in said jacket adjacent to but outside of said form and substantially surrounded by said circulating coolant, to detect the level of molten metal in the fonn;

a billet drive system (16) that is responsive to a drive control signal to vary the rate of removal of the billet from said form;

a source (41) of excitation signals coupled to said inductance detector; and

means (40, 47, 24, 22) coupled to said inductance level de tector for extracting therefrom a signal indicative of the level of molten metal and for determining therefrom a variable drive control signal, and for coupling that control signal to said billet drive system;

whereby a change in the level of molten metal from a predetermined level results in a change in the rate of billet extraction so as to restore the molten metal to that predetermined level. 

1. In a mold level control system for controlling the level of molten metal in a continuous casting mold of the type that has a core defining a shaping surface, which core includes a layer of highly conductive material and which mold receives molten metal from a source and from which a continuous billet of at least partly hardened metal is extracted by a variable speed drive system, the improvement of having: an inductance mold level detector, including at least one inductance coil, which detector is positioned adjacent to but without the mold core, for detecting variations in the level of molten metal therein; means coupled to said mold level detector and to the billet drive system for controlling the speed of the billet drive system in response to said detector so as to vary the speed thereof to maintain, during normal operation, the level of the molten metal within a desired predetermined range; said inductance mold level detector comprising two coils, said one coil and a second coil; and said controlling means including: a signal source coupled to said one coil for exciting it with an ac input signal, means coupled to said second coil for deriving an output signal therefrom which is related to the input signal and to the level of molten metal in the mold; a source of a second signal related to the input signal but phase shifted therefrom, and means coupled to said output signal deriving means and said second signal source for deriving a dc mold level control signal whose amplitude varies in proportion with changes in the level of molten metal in the mold core from a predetermined level therein; whereby the mold level control signal may be employed to govern the operation of motors in the billet drive system to counter changes in the molten metal from the predetermined level.
 2. A continuous steel casting system comprising: a source (12) of molten steel; a mold unit (20) having a generally vertical core including an integral electrically conductive form defining a shaping surface (20A) into which form said source pours molten steel at an approximately constant and continuous rate, and a jacket about said form in which an electrically conducting coolant is circulated and from which mold unit a continuous steel billet is produced; an inductance level detector (30) positioned in said jacket adjacent to but outside of said core substantially surrounded by said circulating electrically conducting coolant to detect the level of molten steel in the form; a billet drive system (16) that is responsive to a drive control signal to vary the rate of removal of the billet from the mold unit; a source (41) of excitation signals coupled to said inductance detector; and means (40, 47, 24, 22) coupled to said inductance level detector for extracting therefrom a signal indicative of the level of molten steel and for determining therefrom a variable drive control signal therefrom, and for coupling that control signal to said billet drive system; whereby a change in the level of molten steel results in a change in the rate of billet extraction so as to restore the level to a predetermined level.
 3. The invention of claim 2, wherein: said inductance level detector within said mold unit includes two coils, a primary coil and a secondary coil; said source of excitation signals are coupled to said primary coil; said means for extracting a signal from said inductance level detector and for determining a control signal therefrom is coupled to said secondary coil and includes means for producing a second signal related to but altered from said excitation signal and means for combining the extracted signal from said secondary coil and said second signal to produce the control signal.
 4. A continuous metal casting system comprising: a source (12) of molten metal; a mold unit (20) having a generally vertical, integral and electrically conductive form, which form defines a shaping Surface (20A) and into which form said source pours molten metal and from which form a continuous steel billet is produced, and a jacket about said form in which an electrically conducting coolant is circulated; an inductance level detector (30), positioned in said jacket adjacent to but outside of said form and substantially surrounded by said circulating coolant, to detect the level of molten metal in the form; a billet drive system (16) that is responsive to a drive control signal to vary the rate of removal of the billet from said form; a source (41) of excitation signals coupled to said inductance detector; and means (40, 47, 24, 22) coupled to said inductance level detector for extracting therefrom a signal indicative of the level of molten metal and for determining therefrom a variable drive control signal, and for coupling that control signal to said billet drive system; whereby a change in the level of molten metal from a predetermined level results in a change in the rate of billet extraction so as to restore the molten metal to that predetermined level. 